A twin study of normal aging will be undertaken in order to identify genetic and environmental contributions to age related changes in selected behavioral, physiological and biological characteristics. A total of 212 pairs of twins aged 60 years and older will be studied over a three year period. It is expected that the sample will contain approximately equal numbers of monozygotic (MZ) and dizygotic (DZ) twins. Potential participants will be identified from the birth records of the state of Minnesota and their current status determined using public information. Once contacted, twins will be asked to participate in a five hour in-home psychological, medical and biological assessment. The various endpoints of the study include factors that, in previous work, have been shown to evidence significant twin similarity, and have been implicated in the normal aging process. These major dependent variables include; i) cognitive functioning as assessed by the WAIS-R, ii) physiological and anthropometric measures including blood pressure, pulmonary functioning, and grip strength, and iii) DNA damage and repair capability as cytogenetically assessed. The in-home assessment also includes a detailed structured interview aimed at measuring those environmental factors hypothesized as relevant to the aging process including measures of i) diet, ii) physical activity, iii) intellectual involvement, and iv) environmental exposure. Analysis of the data will seek to determine; i) the extent to which genetic factors play a role in the normal aging process, and ii) the importance of specific environmental factors like physical activity to normal behavioral and biological aging. An additional objective of the study is to establish and maintain a resource for other current and future studies of normal aging.